USE OF HEAVY RIGID TRUCK-TRAILER COMBINATION VEHICLES
IN WESTERN AUSTRALIA
Vic Johnston
Senior Engineer, Pl~nt and Mechanical
Ma;n Roads Department
and
A1 be rt 0 Ne i 11
Engineer, Vehicle Limits
Main Roads Department
I
USE OF HEAVY RIGID TRUCK-TRAILER COMBINATIONS IN WESTERN AUSTRALIA
ABSTRACT
The road transport industry in Western Australia utilises a
very wide range of vehicle configurations to maximise transport
economies in a very large and sparsely populated State. Whilst the
largest combination freight vehicles, roadtrains, are amongst the
longest and heaviest in the world there are also other unique features
of the Statels fleet. Developments have taken place in the use of 17
m truck trailer combinations with a gross vehicle mass of 58.5 tonnes
and short 18 m fBI Doubles at 63.0 tonnes. These vehicles, which
operate under special conditions, and have proved very popular with
transport operators. The paper details operating conditions and.
vehicle characteristics and compares several key performance factors
for a number of vehicle configurations given a set transport task.
The advantages of IBI Doubles and rigid truck trailers over more
common vehicles are confirmed.
Authors I Name:
V Johnston and A OINeill
Authors I Position
Senior Engineer Plant &Mechanical and
Engineer Vehicle Limits
Main Roads Department of Western
Australia
Waterloo Crescent
EAST PERTH WA 6000
Title of Paper
Use of Heavy Rigid Truck-Trailer
Combinations in Western Australia
INTRODUCTION
Western Australials general freight vehicle fleet is the most
diverse in Australia. The largest combination freight vehicles, road
trains, are amongst the longest and heaviest in the world.
Medium Combination Vehicles in Weste~n Australia operate in the
range from 42.5 tonnes to 64 tonnes gross combination mass and
generally are within the regulation length limit of 17.5 metres. This
development has resulted from the necessity for large quantities of
freight to be transported over long distances efficiently and
economically and is possible because of the unique characteristics of
the road and bridge network and its environment. The experience
Western Australia has gained with this class of vehicle has enabled
its use to be permitted extensively throughout the arterial road
syste~ and in particular within the Metropolitan Region.
This class
of vehicle has great versatility and during the last ten years has
been gaining increasing popularity and acceptance.
Medium Combination Vehicles and Road Trains offer significant
economic advantages due to greater payload per vehicle, but as with
all large vehicles there are disadvantages with their use which arise
from the interaction with other traffic and the permitted geographical
location of the operation of these vehicles. Aspects such as the
percentage heavy vehicles in the traffic stream, total traffic volume
and geographic considerations {e.g. manoeuvrability, terrain, road
"formation and pavement width} make it essential for the movement of
these vehicles to be strictly controlled.
In practice control is effected by permits issued by the Main
Roads Department WA for specified combinations of vehicles to operate
on approved selected routes. Whenever possible loading controls a~
engineered into the vehicle or applied as a condition of the permit to
reduce the possibility of overloading enabling greater flexibility of
movement and enhancing the overall enforcement strategy.
In Western Australia Medium Combination Vehicles (Truck and
Trailer) have been operating for many years. Their use is more
widespread in Western Australia than in other States. The controls
which are applied to the operations of these vehicles have been
progressively evolved in the light of experience.
The IBI double combination commenced operation on selected
routes in the Perth Metropolitan Region in 1983. These early vehicles
were designed and constructed in such a way that it is unlikely that
the regulation axle group load limits on any axle group could be
exceeded. These early vehicles were designed for specific commodities
of known densities and therefore were constructed ensuring regulation
axle masses were adhered to on all axle groups. With the introduction
of the NAASRA 23 m 18 1 Double nationally and the acceptance of general
freight a relaxation of guideline in WA is being considered and the
carriage of general freight is likely to be approved in the near
future.
USE OF HEAVY RIGID TRUCK-TRAILER COMBINATIONS IN WESTERN AUSTRALIA
WESTERN AUSTRALIA - BACKGROUND
The special characteristics of the road transport industry in
Western Australia are partly attributable to the particular geography
and demography of the State. Western Australia occupies one third of
the continent of Australia. It covers an area of over 2.5 million
square kilometres. The population of the State is approximately one
and a half million people which is only 8.5% of the Australian total.
This relatively low figure and the distribution within Western
Australia has been greatly influenced by the vast arid country and the
unique climatic conditions.
The population is concentrated in and around Perth. About 75%
of the State population live within a 50 km radius of the centre of
Perth in an area which is 0.2% of the whole State. Almost 87% of the
population live within the south west corner of the State, an area of
9% of the State total. The population, area, and population density
are shown in Table 1 for the-State as a whole. The population density
figure is one of the lowest in Australia. Rural areas of the State
are even less densely populated. However, the population growth in
the Western Australian rural area between 1971 and 1981 was one of the
highest in Australia.
TABLE 1
Population
Area (1000
Population
Population
DEMOGRAPHIC INFORMATION
(1000 persons)
km 2)
Density (persons/km 2 )
Growth
1.5
2 527
0.6
2.11
Road transport has become and will continue to be the dominant
force in the development and servicing of a major portion of the State
as the railway system is confined to specific areas and concentrates
on a limited range of bulk commodities.
WESTERN AUSTRALIAN VEHICLE FLEET
Approximately 90% of road vehicles in Western Australia are
cars and light commercials. These provide the principal demand for
V JOHNSTON AND A O'NEILL
road facilities. However, it is the larger and heavier commercial
vehicles which determine the design requirements for roads and
bridges. Whilst comprising only 10% of the vehicle fleet they are
responsible for some six thousand billion tonne-kilometres of travel
on the State1s road network annually.
Historically, the development of the road .transport fleet has
progressed from two horsepower vehicles with gross combination mass of
2 tonnes, to vehicles of 500 horsepower or more with a gross
combination mass of up to 138 tonnes. The benefits to be achieved by
permitting vehicles to carry greater loads are recognised and have in
fact been achieved by carrying increased gross loads over a greater
number of axle groups, but with all axle groups loaded to the
regulation limits.
Therefore, as shown in Figure 1, a diverse range of commercial
vehicles has developed in Western Australia, many of which are
permitted to operate well in excess of the regulation gross mass limit
of 38 tonnes. For example, combinations consisting of a 4-axle rigid
truck plus a 4-axle dog trailers, and having a gross combination mass
of 58.5 tonnes, are widely used on a number of approved routes
throughout the State. Also permitted to operate on selected routes
throughout the State are road trains, consisting of combinations of
normal regulation vehicles - either a rigid truck plus two or three
trailers, or an articulated vehicle plus one or two trailers.
The most common road train operating in Western Australia is
the "double bottom", consisting of an articulated vehicle plus one
trailer and operating with a gross combination mass of approximately
72 tonnes, with an overall combination length of up to 33 metres.
"Triple bottoms", consisting of an articulated vehicle plus two
trailers, with a gross combination mass of approximately 105 tonnes,
are also common in northern parts of the State.
ADMINISTRATION OF EXTRA MASS AND OVERDIMENSIONS SYSTEM
Regulation Mass Limits
Legally enforceable mass limits provide a uniform basis for
vehicle manufacturers to design vehicles, road designers to design
roads and bridges on which vehicles will operate, and give transport
operators the opportunity to utilise vehicles which will optimise
their operations.
TYPICAL HEAVY COMMERCIAL VEHICLE TYPES
IN
WESTERN AUSTRALIA
c
V>
~
(Regulation Lenglh 11 m mu.)
(i)
no
13.9
•
00
••
CD Cl
00
11 m
21.9
••
..
(Regulallon Lengl h 17 m mIX.'
Gross Mass (I)
CV
@
0)
I'T1
ARTICULATED VEHICLE
~
o •
LJ
0
••
@
CJ
0
®
(l
25.5
"Tl
Single ule, single lyre
5.4 I
Tlndem ule, single tyre
9.0 t
•
Single ule, dUll tyres
8.5 t
••
Tlndem ule, dual tyres
16.5 t
• • • Trlule, dual tyres
18.0 t
For types
10 @Irlule groups may be
used on the rear ot all trailers & with utr.
mass permits tor 21 lonnes per group the
gross mass can b. IS shown.
~~-=
sm
00
39.9 (42.9)·
*
43.5 (46.5)·
00 ••
I'T1
o
38.4
•••
:c
LEGEND
30.4
••
••
o
Gross Mass (I)
• ••
@
SCII.
______c:====..____
~---1.Lm
)::-
<:
-<
::0
.......
G)
.......
Cl
~
::0
C
Cl
A
I
~
--i
::0
)::-
.......
Gross Mass (I)
CD
CD
--
•
Q-- .. ~ ..
CD Cl
0
CV {]
00
CD
~
~-
.. --.- ••
•• - .
••
.. --.. ..
22.4
@
n
00
f.-
38.4
..
ROAD TRAINS (TRUCK PLUS TRAILER TYPE)
Rigid
Truck
Rigid
Truck
@
00
r
•••• ....
Trailer 1
91.5 (100.5)·
.. ..
Z
)::~
Trailer 2
Trailer 3
124.5 (138.0)·
•• ••
o
••
:e:
mu. - - - - - - f-------,~
la rn",lmu-m length 01 20 m Is possible)
58_5
_ 17_111_ __ __ -j
@
Ar"liculated Vehicle
f------------------------
FIGURE 1
~
Trailer 1
(8
71.4 (80.4) *
••
-------1
0.. .... ....
(l
I'T1
V>
Trailer
••R!11 ••
z
.......
z
ROAD TRAINS (ART'ICULATED VEHICLE PLUS TRAILER TYPE)
50_5
.......
o
V>
._ _ _ ~~ IT! ______________________ ~ _ _5CUn_-l
Articulaled Vehicle
3:
OJ
46.9
@ (l
o
.......
---1- _4Lm__ -,
30 m
::0
Cl
Trailer 2
Trailer 1
•• ••••
~
I'T1
Gross Mass (t)
50 m max.
minimum length 01 33 inis -possiblej----
I'T1
::0
Z
)::-
C
V>
~
Trailer 2
104.4 (117.9)*
••
------1
TYPICAL HEAVY COMMERCIAL VEHICLE TYPES
::0
)::-
r
.......
)::-
V JOHNSTON AND A OINEILL
All motor vehicles using public roads throughout Western
Australia are licensed by the Western Australian Police Department1s
Licensing and Services Division under powers prescribed in the Vehicle
Standards Regulations (VSRs) of the Road Traffic Act 1974-1976.
Certain of these VSRs stipulate the mass limits at which a vehicle may
operate on any road in Western Australia. These regulation mass
limits are shown at Table 2.
It is worth noting that in 1985 a national Review of Road
Vehicle Limits (RoRVL) was undertaken by the National Association of
Australian State Road Authorities (NAASRA), its purpose being to ...
IIReview the mass and dimension limits applying to vehicles using
Australian roads, with the objective of enabling the Road Transport
Industry to improve its economic viability.1I
TABLE 2
PROPOSED CHANGES TO AXLE AND
GROSS MASS LIMITS - WESTERN AUSTRALIA
MASS LIMIT (Tonnes)
AXLE GROUP
Single Axle, Single Tyres
Single Axle, Dual Tyres
Twin Steer, Load Sharing
Twin Steer, Non-Load Sharing
Tandem Axle, single tyres
Tandem Axle, Dual Tyres
Tandem Axle, single and dual tyres
Triax1e, dual tyres
Gross Mass
SOURCE
EXISTING
PROPOSED
5.4
8.5
6.0
9.0
11 .0
10.0
9.0
10.0
16.5
12.0
10.0
11 .0
16.5
18.0
13.0
20.0
38.0
42.5
Review of Road.Yehicles Limits, NAASRA, 1985
The RoRVL study noted that options for increases in mass limits
were constrained by the capacity of the community to fund the
increased cost of road damage from heavier vehicles. RoRVL
recommended minor increases in vehicle mass limits to assist industry
USE OF HEAVY RIGID TRUCK-TRAILER COMBINATIONS IN WESTERN AUSTRALIA
productivity, but which also reflected current financial constraints
on road and bridge works. For Western Australia, legislation to
increase certain mass limits in accordance with the RoRVL
recommendations is under consideration. The changes proposed are
shown in Table 2.
The Permit System
Under the Western Australian Vehicle Standards Regulations the
Commissioner of Main Roads may issue a permit to allow a licensed
vehicle to be operated when it does not comply with the regulation
limits for dimension or mass, or both. Extensive use is made of the
permit system which enables concessions to be made to the transport
industry where conditions allow. The availability of these
concessions gives the transport industry a great deal of flexibility
in its choice of the type of vehicle for a particular task. It
enables the transport industry to respond to the transport needs of
the community and whenever possible it allows the most economic
transport method to be employed.
Permits are concessions and not rights. They are made
available subject to a variety of conditions, and failure to comply
with the permit conditions will render a permit liable to
cancellation. Loss of permit concessions by an operator will have far
reaching effects on his business and this control procedure is very
effective in ensuring that operations are conducted in accordance with
permit conditions.
Statutory vehicle limitations in effect serve as the lowest
common denominator. In certain circumstances special dispensation to
exceed this limitation for particular haulag~ operations can be
justified. The indiscriminate issue of permits could however result
in financial and social burdens on the community. Therefore permits
are only issued after taking into regard all the relevant factors such
as safety, likely damage to road and bridges, equitable use of the
road system, environmental impact and compatibility with national
standards for transport economies. Routes with large traffic volume
and percentage of heavy vehicles stand to contribute greater marginal
benefits if the road pavement is constructed to withstand these
additional loads.
A total of approximately 20 000 permits were issued in 1987/88
of which 460 applied to truck/trailer combinations and 1700 were Road
Train Permits. Total permit fees collected from all permits issued
amounted to approximately $650 000.
V JOHNSTON AND A O'NEILL
VEHICLE CONFIGURATIONS
Rigid Truck and Trailer Combinations
The various combinations discussed in this section are shown as
Vehicle numbers 4 to 8 of Figure 1. In Western Australia the
operation of this class of vehicle combination has grown quickly.
Their use has been able to be permitted because they perform on the
road system very well from many points of view, their turning swept
path is better than that of a standard 17 m articulated vehicle as
shown in figure 2 and they also offer a dramatic increase in payload
over a standard articulated vehicle. Their gross combination masses
range from 48.0 to 60.5 tonnes and with only a very limited number of
exceptions for very specialised applications confined to timber
country, they are within the overall combination length of 17.5 m.
For those operating at gross combination masses of up to 53.0
tonnes, Extra Mass Permits have been issued for many years to operate
over a major portion of the road system including the Perth
Metropolitan Area. Apart from restrictions of route, there are no
limitations applied to the operations of these combinations. The
network of routes which these vehicles are permitted to operate is
only limited by some weak bridges. As a further condition of
operation and permit it is a requirement that these vehicles be
weighed at some point of the journey and accessibility to"the
weighbridge dockets by the Main Roads Department is a permit condition.
In Western Australia a distinction has been drawn between
single steer units and twin steer units. Single Steer units with
tandem drives are permitted regulation axle loadings, i.e. 5.4 tonne
and 16.5 tonne respectively with a total mass of 21.9 tonne. These
units however when used in a truck trailer combination are restricted
to a maximum of 25 tonnes for the rear trailer permitting a total
gross combination mass of 46.9 tonnes for this configuration. The
limiting factor being the mass being towed. If no restriction was
applied a 50% increase in towed mass could be achieved.
It can be appreciated that as the mass being towed increases
significantly above the mass of the towing vehicle the situation
arises when the front vehicle behaviour is determined by the rear
trailer. In terms of operating in a metropolitan environment with
high traffic volumes and high pedestrian activity this type of vehicle
performance reduces road safety and introduces unnecessary hazards.
For these reasons the gross mass of the rear trailer is restricted to
25 tonnes irrespective of the trailer axle configuration.
RAD
--- ---
17()
t-
- Gl
ia..
5c
0-0
VU4.G
• ,t.94~~~
..
/7-0
rQ tJ~.&os
t
-Q
-- - ----
.....-. . . - .
~
RIGID TRUCK & TRAILER
ARTICULATED VEHICLE
~
17-0
&et
..-.
·1
1
00
~·~I. 5·0 ~l·
- - - --
S{ALE
0
0'
FIGURE 2
-1
ocr
CANADIAN 'B' TRAIN
COMPARISON OF SWEPT PATH PLOTS
-
.........................
12 m
1 : 200
V JOHNSTON AND A OINEILL
The twin steer vehicle is permitted a gross vehicle mass of
25.5 tonnes with a further allowance of 1 tonne if the front
suspension is certified as a load sharing suspension. In this
configuration the rear trailer is allowed full axle loadings. It is
recognised that the ratio of the towing vehicle to the mass being
towed increases however the twin steer unit offers the added safety
and security of a tandem front steer system. These vehicle
combinations are extensively used throughout Western Australia but are
restricted to bridge strength evaluated routes. Each application
requires agreement from the relevant Authorities which control the
routes over which the vehicle requires approval and a further bridge
assessment is carried out to assess any structural inadequacy.
Since 1978 there has been a growth in the issue of Extra Mass
Permits for the four axle truck and four axle trailer combination with
a gross combination mass up to 60.5 tonnes now allowed on permit.
Approximately 460 permits were issued during 1987/88 for truck
trailers.
Permits for these vehicles have been issued strictly in
accordance with the following guidelines cartage of a nominated commodity between fixed terminal points
weighing facilities to be used en route for accurate
determination of permitted laden masses
routes to be approved by the MRD, i.e road conditions and
bridge capacities to be adequate
Local Government approval usually required for permit
operations outside of the Perth Metropolitan Region.
Where further conditions are imposed by the Local Authorities the
permit system is flexible enough to include these requests and
endorsements are added to the relevant permits. Currently no maximum
road speed is imposed on these units and the only pre-requisite to the
issue of a permit is the qualification that the hauling unit be rated
with the appropriate gross combination mass.
In addition to the limits outlined above, there are regulation
mass limits for axle group loadings. Again, the Commissioner may
issue Extra Mass Permits to exceed these limits, but in normal
commercial vehicle operations such permits are only issued for triaxle
groups. The regulation limit for such groups is 18 tonnes but permits
are issued for 20 tonne loadings. The introduction of RoRVL
recommendations will supersede the requirement for pennits in this
case.
USE OF HEAVY RIGID TRUCK-TRAILER COMBINATIONS IN WESTERN AUSTRALIA
The Commissioner has the power to withdraw permits. For
example permits can be withdrawn for violation of permit conditions
and violation could include the use of unauthorised routes and
overloading. Because of this power, the escalation of capital
equipment cost of vehicles with increasing capacity and the road
transport industryls general view that permits are a concessional
privilege (and a means of ilnproving the efficiency of their
operations), vehicles operating under permits issued by the
Commissioner have a better record for compliance with regulation axle
group loadings than normal as of right vehicles. In essence, they are
less damaging to the road system. In addition, these higher capacity
vehicles are generally operated by the more competent drivers and are
better maintained.
The IBI Double
During the 1970s, although the ISO rating for the maximum laden
mass of 6 m ISO seatainers was 21 tonnes, the majority of seatainers
being handled in Western Australia were well below this maximum. The
use of rigid truck and trailer combinations as shown at Types A & B,
Figure 3, with a permitted gross combination mass of 46.9 and 50.5
tonnes, and to a lesser extent, articulated vehicles as shown at Type
C, Figure 3, with a gross mass of 38 tonnes, became more relevant for
the movement of 6 metre long ISO seatainers two at a time. However,
as the number of seatainers with laden masses of up to about 21 tonnes
increased, the incidence of overloading on rigid truck and trailer
combinations escalated to the extent that it became a real concern to
both the administering authority and responsible transport operators.
Table 3 shows some typical loading patterns which can result
from the use of rigid truck and trailer combinations for the transport
of seatainers two at a time. These results demonstrate the inability
of this type of combination to equitably distribute loading over the
supporting axle groups' this produces a high potential for overloading
and a reduction in transport efficiency.
In
container
IBI Train
operate a
examining alternative vehicle configurations for this
task a suitable combination type appeared to be the Canadian
concept. In April 1983 a permit was issued to a company to
Canadian IBI Train on a trial basis.
In Western Australia because of the sensitivity of roadtrain
activity the terminology IBI Double has now been promoted instead of
IBI Train.
V JOHNSTON AND A Q'NEILL
TYPE A - TRUCK & TRAILER COMBINATION WITH PERMITTED
GROSS COMBINATION MASS OF 46.9 TONNES.
TYPE B - TRUCK & TRAILER COMBINATION WITH PERMITTED
GROSS COMBINATION MASS OF 50.5 TONNES.
TYPE C·
ARTICULATED VEHICLE WITH MAXIMUM
GROSS COMBINATION MASS OF 38.0 TONNES.
---
TYPE OF
COMBINATION
THREE AXLE RIGID TRUCK & THREE AXLE TRAILER
01
oc)
0
I
1
0
FOUR AXLE RIGID TRUCK & FOU R AXLE
QI
I
00
00
l
I00
TRAIL~ R
I
00
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AXL-&
AXU
6~CVr-
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~ll"1lr
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L,OAO/~&!
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F£OA/T
.sTeeR
ORIVC
T£Alt£;R
TRAIL&~
6·4-
16·6
8·6
/6·5
6·/
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7· 'I
6· 7
22·/
'·7
7.J
24·1
ST&&R
&£oup
DRive
t
CJ401A../t:-
F£.OMT
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TtI!.A / t., &£
T£AIL-C/2.
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13'4-
7·'
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20·4-
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S&A TAIM&('S
2,1t 1/
TOMt/{,
SCATAIA./t;RS
21(24 TCWd
10'~
S&ATAlN~S
1...-
FIGURE 4
-
-
-----
---
.. _------
TRUCK TRAILER LOADING ANALYSIS
/4·6
/5·7
V JOHNSTON AND A OINEILL
The overall length of this combination exceeded the regulation
limit of 17 metres by up to 1 metre thus giving a maximum overall
combination length of approximately 18 metres. However, because the
swept path characteristics of the combination were better than a
regulation 17 metre articulated vehicle, see Figure 2, this slight
increase in length was not considered to be of any significance.
In general, routes were confined to major arterial roads
connecting ports with industrial areas and major rural centres
together with essential access to and within industrial areas. In all
cases operations were only permitted on routes approved by the Main
Roads Department.
In the initial stages it was considered that the use of these
units should not be permitted for other commodities where variability
of loading was considered to be a problem however the possibility of
relaxing this requirement is currently being investigated and it is
likely that the carriage of general freight will be approved in the
near future.
In terms of the operational characteristics of the 18 1 Double
all drivers have reported a feeling of confidence in the stable
handling characteristics, particularly when compared with the common
rigid truck and trailer combinations. This no doubt is a reflection
of the difference in coupling mechanisms through two fifthwheels in
the case of the 18 1 Double compared with one fifthwheel and a pin type
coupling with the rigid truck and trailer. The longitudinal torsional
stability imparted through the two fifthwheel connections produces a
more direct feel by the driver for the behaviour of the vehicles being
towed. In Western Australia these units have an excellent road safety
record with no reported accidents.
Acceptance by the general public has been ratified by no
complaints being received from other road users or the public
concerning the operation of these vehicles. This is not unusual in
that 18 1 Doubles are very similar to rigid truck and trailer
combinations which have been prevalent in both the metropolitan and
rural areas of WA for many years.
In regard to overloading, 18 1 Doubles are subject to the same
field surveillance, risk of detection and prosecution for overloading
as other commercial vehicles. Having regard for the generally strict
controls over container loadings, particularly the upper limit of 24
tonnes, the risk of containers exceeding a laden weight of 24 tonnes
USE OF HEAVY RIGID TRUCK-TRAILER COMBINATIONS IN WESTERN AUSTRALIA
is not high. Also the IBI Doubles can more adequately distribute the
total payload over the supporting axle groups than the conventional
rigid truck and trailer combinations; this feature will considerably
reduce the risk of excessive overloads which have become more
prevalent with the rigid truck and trailer combinations.
Currently Western Aust~alia has 20 IBI Double configurations at
18 m long and 5 at 23 m long. All vehicles are subject to the
National Association of State Road Authorities (NAASRA) Speed and
Gradeability assessment.
With the introduction of the new NAASRA 23m IBI Double specific
routes on the outer perimeter of the Metropolitan area have been
designated and in this regard because the traffic is generally lighter
no distinction has been drawn in respect of the advantages of twin
steer versus single steer.
VEHICLE COMPARISON - ECONOMIES OF SCALE
There can be little doubt regarding the economies in road
transport costs to be achieved by using large combination vehicles.
For example, an analysis of the task of carting 100 000 tonnes of
cargo over 160 kilometres (100 miles) in different types of
combinations and assuming no backloading, is shown in Table 4.
Included in the performance measures in this table is the
concept of Equivalent Standard Axle (ESA) which is a measure of the
damaging effect of a Standard Axle. The various other commonly used
axle group configurations all have loadings at which they have the
same damaging effect as a standard axle, and when they are laden to
the regulation limit the damage they inflict on the road can be
measured in multiples of standard axles.
It also follows that each axle group when operating at its
regulation limit has a different Road Damage Factor (RDF) measured in
ESAs/tonne. The relationships between the axle groups, their
regulation load limits, their Standard Axle Equivalencies and their
Road Damage Factor is shown in Table 5.
V JOHNSTON AND A Q'NEILL
TABLE 4
VEHICLE COMPARISON FOR SET TRANSPORTATION TASK
100 000 TONNES TRANSPORTED OVER 161 KM
RIGID
TRUCK
VEHICLE TYPE
AXLE LOADINGS
LADEN MASS
5.4
8.5
0
0
SEMI
TRAILER
5.4
0
16.5 20.0
00
000
'B'
DOUBLE
RIGID
TRUCK/TRAILER
9.0 16.5 16.5 16.5
00
00
00
00
ROAD TRAIN
DOUBLE BOTTOM
9.0 16.5
20.0 16.5
00
000
00
00
5.4 16.5
0
00
20.0 16.5 20.0
000
00
13.9
41.9
58.5
62.0
78.4
UNLADEN MASS
4.9
15.9
19.5
20.0
28.4
PAYLOAD
9.0
26.0
39.0
42.0
50.0
22 224
7 692
5 130
4 762
4 000
ESA/VEHICLE - LADEN
- UNLADEN
2.15
0.03
4.53
.07
7.50
0.241
6.70
.48
8.07
0.71
TOTAL ESA'S - LADEN
- UNLADEN
23 891
334
17 422
269
19 238
618
15 953
1 143
16 140
1 420
OVERALL TOTAL ESA'S
24 225
17 691
19 856
17 096
17 560
29.0
52.0
56.0
58.0
63.0
3 578 064
1 238 412
825 930
766 682
671 048
1 027 638
643 974
462 521
444 675
422 760
208 906
222 298
200 070
195 242
213 600
4.18
5.70
5.20
6.27
6.20
TOTAL TRIPS
(LADEN & UNLADEN)
FUEL CONSOMPTION
LTS PER 100 KM
KILOMETREAGE
FUEL CONSUMPTION
TOTAL TONNAGE
PAYLOAD
TONNE/ESA
LTS
('\""
USE OF HEAVY RIGID TRUCK-TRAILER COMBINATIONS IN WESTERN AUSTRALIA
TABLE 5 : RELATIONSHIP BETWEEN AXLE MASS AND ROAD DAMAGE
Axle Group
Single steer
-2 tyres
Tandem steer
2 tyres/ax1 e
Single axle
4 tyres
Tandem axle
4 tyres/axl e
Triax1e
4 tyres/axl e
Loading in Regulation Standard Axle
tonnes at Mass Limit Equivalencies
(ESAs)
(tonnes)
which equiv.
to 1 Std Axle
Road Damage
Factor in
ESA/tonne
5.4
6.0
1.524
0.254
9. 1
11 .0
2.135
0.194
8.2
9.0
1.451
13.6
16.5
2. 167
o. 161
o. 131
18.5
20.0
1.366
0.068
Looking at the two extremes from Table 4, if the task is
undertaken utilising a 2-axle truck with an allowable gross mass of
13.9 tonnes and payload of 9.0 tonnes, the total trips necessary,
laden and unladen, would be 22 224, consuming 1 037 638 litres of fuel
and subjecting the road to the equivalent of 24 225 standard axle
loadings.
In comparison, an 8-wheel rigid truck with a 4-axle dog
trailer, with a gross combination mass of 58.5 tonnes and payload
capacity of 39 tonnes, would accomplish the task in 5 130 trips,
consume 462 521 litres of fuel and subject the road to the equivalent
of 19 856 standard axle loadings.
Comparing these two examples and taking the cost of fuel to be
about 55~ per litre savings in fuel cost alone would be about $316 000
for this particular task. There would also be a reduction of 37% in
road wear.
V JOHNSTON AND A O'NEILL
From the foregoing, it is readily apparent that the potential
benefits of using vehicle combinations with increased payloads, but
with the increased payloads being distributed over a greater number of
axles, with each axle complying with the regulation limits, are :A considerable saving in fuel
A reduction in the number
o~
vehicle movements, and
A reduction in pavement deterioration.
In order to gain such public acceptance, all of those connected
with the road transport industry - designers, builders, operators and
administrators - have a real responsibility to achieve in a sensible
and acceptable manner innovations in commercial vehicle design and
operation which will not only be of benefit to the community as the
end recipients of transport costs, but be acceptable to the community
as road users.
SUMMARY & CONCLUSIONS
Road transport is usually characterised by its dual ownership
in Western Australia. The road network is financed and administered
by the Main Roads Department and Local Government Authorities, whereas
the vehicles using it are largely in private ownership. In the
absence of unified ownership and control, regulation of the mass and
dimension limits for commercial vehicles is necessary to achieve a
balance between the potential of vehicles to carry large and heavy
loads and the capability of the road network to accommodate such
vehicles.
Restrictions on the mass of vehicles and axles are applied to
ensure the safety of other road users and to ensure an appropriate
balance between transport economies and the cost of road construction
and maintenance. There are, however, occasions when it is necessary
and acceptable to exceed regulation vehicle mass limits. Large
combination vehicles, carrying increased pay loads over a greater
number of axles, but with all axles complying with regulation mass
limits, can be shown to reduce pavement damage, fuel consumption and
the number of vehicle movements. In recognition of such benefits,
permits are issued within prescribed limits where road and traffic
conditions permit.
USE OF HEAVY RIGID TRUCK-TRAILER COMBINATIONS IN WESTERN AUSTRALIA
Consequently a diverse vehicle fleet has developed in Western
Australia, enabling the transport industry to respond to the transport
needs of the community in a flexible and economic manner. However,
such permits are subject to a number of strict conditions to ensure
their safe operation with minimum impact on the road network.
Control of axle load limits is of fundamental importance in the
protection of road and bridge assets, as well as for the safety of
other road users. In Western Australia, control of axle load limits
is based upon identification of a network of heavy haulage routes,
field enforcement of vehicle and axle mass limits, and a system of
financial penalties to discourage overloading. A statewide network of
heavy haulage routes, designed to accommodate large, heavy loads,
provides a basis for the safe and efficient movement of such vehicles
between major industrial and resource centres. Field enforcement of
vehicle and axle mass limits in Western Australia is undertaken by
mobile police crews equipped with thin plate portable weighing
scales. For vehicles found to have exceeded the mass limits, (after
allowing weighing and administrative tolerances), a sliding scale of
financial penalties is applied. Vehicles operating under permit are
also liable to have permits cancelled for serious violations of their
permit conditions.
Development of the abovementioned monitoring and control
systems in Western Australia has enabled a diverse range of transport
vehicles to operate throughout the State in a safe and efficient
manner, whilst at the same time safeguarding the substantial public
investment in the State's road network.